In a DMOS transistor, the channel region of the transistor is inverted by an appropriate voltage applied to gate insulated from and overlying the channel. The inverted channel forms an ohmic path between two semiconductor regions of a same conductivity type separated by the channel.
A major contributor to on-resistance of low voltage DMOS transistors is channel resistance. For a low voltage DMOS transistor it is therefore desirable to fabricate the DMOS transistor so as to have a short channel so that the channel resistance does not unduly contribute to the on-resistance of the device.
In the formation of conventional DMOS transistors, it is well known to use a formed gate as a mask when doping a surface of a silicon substrate to form the body region and source region. This enables the body region and source region to be self-aligned with the gate. This well known process is generally shown in FIGS. 1-3, wherein FIG. 1 shows P-type boron ions 2 being implanted in N- epitaxial layer 3, formed over N+ substrate 4, to subsequently form the body regions of a vertical DMOS transistor. As seen in FIG. 1, gate 5 and thick oxide regions 10 provide a mask during the implantation of the P-type ions so that the body regions will be self-aligned with the gate. Previously formed P+ regions 12 act as contact regions for the body regions.
The implanted P-type ions are then driven in at a high temperature to achieve the desired diffusion of the P-type ions, shown in FIG. 2, in N- epitaxial 3 and under gate 5.
In FIG. 2, a conventional masking and etching step is used to expose surface portions 16 of P- body regions 14 for implantation or diffusion thereinto of N-type impurities, such as phosphorous or arsenic, using gate 5 and etched thick oxide 17 as a mask.
In FIG. 3, a drive-in step is then conducted to form relatively shallow N+source regions 18 within P- body regions 14 so that N+source regions 18 extend under gate 5 but remain totally within P- body regions 14.
As previously stated, for low voltage DMOS devices, the channel regions 20 under gate 5 are desired to be short to achieve a low on-resistance while still preventing breakdown of the narrow P- body regions when the DMOS transistor is in its off condition.
One way to achieve a short channel is to form a shallow P- body region by implanting P-type ions at low energy and then performing a short drive-in step, or a drive-in step at a relatively low temperature. The limited drive-in of the P-type impurities results in a limited diffusion of the P-type impurities under the gate, while enabling good control of the resulting impurity concentration of the P-body region.
A subsequent implantation or diffusion of N-type impurities into the shallow P- body region and drive-in of these impurities will result in a short channel, since the diffusion of the P-type impurities under the gate was restricted by the limited drive-in of the P-type impurities.
Using this prior art method of forming a shallow P-body region results in the shallow P- body region having a relatively high resistance Rb as compared to deeper body regions formed using the same impurity dose.
FIG. 4 shows a partial cross-section of a DMOS transistor with a shallow P- body region 26 and a short channel 28. Since, typically, N+ source region 30 and P-body region 26 are shorted together by a source electrode (not shown) to prevent the PN junction of the P- body region and N+ source region from becoming forward biased, this high resistance of the shallow P- body region 26 causes a voltage differential across P- body region 26 when any current flows through P- body region 26. If the current is large enough, the voltage differential may be sufficient to turn on the parasitic NPN bipolar transistor comprising N+ emitter 30, P- base 26, and N- collector 3. This high resistance of shallow P- body region 26 also undesirably contributes to a high beta of the parasitic NPN bipolar transistor and the increased likelihood of latch-up of the device.
A process is needed for fabricating a low voltage rugged DMOS transistor with a short channel which enables a relatively deep P- body region to be formed while using the gate as a mask for self-aligning the P- body region.